It is an aerodynamic feature of modern multiengine passenger aircraft to have larger rudder surfaces. This feature is typically necessary in the event one of the engines of the aircraft becomes nonoperational. The remaining operating engines produce a difference in thrust between the left and right sides of the aircraft which cause the aircraft to yaw. By the term "yaw" it is meant that the longitudinal axis of the aircraft is angled to the left or right of the direction of flight; the angular difference between the aircraft heading and the aircraft direction of flight being known as "sideslip angle". The yaw condition is counteracted by the application of opposite rudder by the pilot which reorients the aircraft heading so that the longitudinal axis of the aircraft is aligned with the direction of flight.
It is furthermore a control feature of these aircraft to permit them to be intentionally yawed by the deflection of the rudder. Typically, the pilot will intentionally yaw or sideslip the aircraft just prior to touchdown on landing when there has been a crosswind during the approach. This is because during the approach the aircraft is typically headed in a direction toward the crosswind so that the aircraft will fly a path over the ground in alignment with the runway. However, just prior to touchdown the pilot applies rudder to align the aircraft longitudinal axis with the runway so that during touchdown and rollout the aircraft remains on the runway.
When the rudder is deflected to produce an intentional sideslip, such as during a crosswind landing, the aircraft is caused to rorate about its roll axis due to aerodynamic forces from what is known as "dihedral effect". In order to counteract the rolling of the aircraft, the aircraft pilot applies opposite aileron. It is usually a requirement that the aircraft have sufficient aileron capacity to counteract this rolling moment due to deflection of the rudder in order to maintain the aircraft in level flight.
However, instances may occur when the maximum counteracting rolling moment which can be produced by the ailerons barely counteracts the rolling moment generated by the application of maximum rudder. It is desirable therefore, to provide the aircraft pilot with a warning when the sideslip angle exceeds a selected quantity so that the pilot is aware when the aircraft is approaching a yaw condition which may require maximum counteracting aileron.
Various systems for notifying the pilot of aircraft flight parameters have been disclosed. For example, in U.S. Pat. No. 3,792,426 by Ravenelle et al, there is disclosed apparatus for vibrating the air bladders of a pilot's anti-G suit to warn him of excessive G forces.
Harcum, in U.S. Pat. No. 2,682,042 discloses a signal feeler device associated with the control wheel of an aircraft and which includes a projecting portion which is moved relative to a reference surface to provide the pilot with a tactile indication of aircraft operating conditions.
Hirsch, in U.S. Pat. No. 3,157,853 discloses apparatus for generating linear signals corresponding to aircraft movement along the pitch, roll or yaw axes, as well as rotational measurements of movement about these axes, and for providing tactile information to the pilot which corresponds to the generated signals in order to aid the pilot in controlling the aircraft.
Appartus for providing a tactile sensation to the legs of a pilot as a tactile indicator of changes in pitch, roll or yaw is disclosed by Bennett in U.S. Pat. No. 1,941,533.
A system for alerting a pilot when the aircraft is in an uncoordinated turn is disclosed by Vavra in U.S. Pat. No. 4,484,191, in which the position of a ball bank indicator is sensed, and vibrators are operated in the pilot's seat cushion to correspond to the left or right position of the ball indicator.
Also, in U.S. Pat. No. 2,287,621 by Reichert et al, there is disclosed apparatus for vibrating the rudder pedals of an aircraft to indicate the approach of the aircraft to a stall condition. In Gilson et al, U.S. Pat. No. 4,195,802 there is disclosed a slide member which is operatively connected to the cyclic control of a helicopter and which moves relative to the cyclic control in response to command signals from a flight director to provide the pilot with tactile information of helicopter flight data.